So this works, putting all tier 3 talents into one button without needing to change bindings each time.

However it doesn't update the graphic. That is, it only displays Sacrifical Pact's cooldown (or whichever one you put first), instead of which talent you actually have. The same problem occurs with tier 2 talents.

I noticed other classes don't have this issue. Swapping from Frost Bomb to Nether Tempest on my mage will update all keybinds and UI elements automatically.

Is there a way to fix this macro so that it displays the appropriate ability cooldown?

download the addon, login, make a macro called tier_3, put in /cast tier_3, put it on your bars.
You can do that with all the talents (from tier 1 to tier 6) and they will automatically change whenever u swap between talents.. it works for all classes and talents